Hello. I would take your doggy to your vet asap, or to an emergency service asap if your vet is not open. My main concern is that he may have a Gastric Dilatation/Volvulus (GDV), where the stomach bloats up and twists around itself, then gets stuck in that position. This very quickly becomes a life threatening situation as he can go into shock and organ failure. Also it's very painful! Other possibilities could be a gastritis or oesophagitis due to something he has ingested, pancreatitis or a foriegn body stuck somewhere in his gut. Either way, if his breathing is affected, I strongly advise you arrange to have him seen by a vet straight away. Hope it all works out ok!
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